Projection lamp illumination system for lenticulated



July 29, 1952 E. GRETENER Filed March 24, 1950 I5 Sheets-Sheet l Fig.10

NVENTOR R .E4 m, MDN O, E v @MW m A m. m D l n; w .m l+|||| lll ,l M M S M ll. 4| l L er M/ mm n L l 7 lll-I l 2 mm u. s l IT n T M5 l, 4, L.. 1 a Il U 1 lull! C n; E ew W n m a m F Y L B m RF am Nm Ew UN u. Rnlv d l GM M, Em u 7 M r 9 n w u D.. um rn M u.. 3 M w ONU w mw 2 @.4. 5 O 2 wr 9 R l DL um M 9 2 M my .l .J F

Wn. ...C 3 5 O1 m l N 8 m MED m 4MM olmME w OVW .wlw :GRU 1 A l IZ ZFn ,/v Vmm ,W\ Ms ,VA l 3 3,7. T C N 5 n B My, R Rm m. Em y ma B Rw GH .A Em w L 1 m. P M A L N0 mm.. mm1 9mm 3 lnxh m 9 a 2 M .Y d, n J F Patented July 29, 1952 PROJECTION LAMP ILLUMINATION SYSTEM FOR LENTICULATED FILM Edgar Gretener, Zurich, Switzerland, assignor to Ciba Products Corporation, Kimberton, Pa., a. corporation of Pennsylvania Application March 24, 1950, Serial No. 151,642 In Switzerland March 29, 1949 (Cl. Sti- 24) 25 Claims.

This invention relates to illumination systems for colored motion pictures, more particularly to illumination systems for the projection of colored motion pictures by lenticulated film.

Illumination systems for the projection of colored motion pictures by the additive method of color mixture hitherto known, employ absorption color filters in order to produce the required light beams of different color. The absorption filters are inserted in the light path either between the light source, preferably an arc lamp, and the picture window or between the picture window and the projection objective. Such illumination systems do not yield illumination of the projected picture with utmost brightness and efficiency as the use of absorption filters implies high losses of light. l

Recently a new type of color filter has become known, producing the filter effect not by absorption, but by discriminately transmitting or refiecting different parts of the light spectrum, thus effecting spectral decomposition of white light. In such reflexion filters or dichroic mirrors a preselected part of the light spectrum can be singled out by a partly reflecting surface whereby the filter has the characteristic of transmitting only said part of light spectrum, but refiecting all other parts, or inverse. It is consequently possible by the application of consecutive reflexion filters to decompose white light into several parts of differently colored light which all can be utilized. In case of the projection of colored motion pictures the light will preferably be decomposed into red, green and blue. Such dichroic mirrors or reflexion color filters practically incur no loss of light, as the cause of loss in the customarily employed color filters, viz absorption of the not transmitted parts of the spectrum, is removed. The full exploitation of the advantages offered by reflexion.v 40

filters in customary projection systems is, however, greatly impaired or even rendered impossible by the optical properties of the customarily used one-sided lenticular film.

Another disadvantage is inherent in hitherto` The present invention has the object to over- 5,5

come all such difficulties by eliminating the defects inherent in known forms of projection systems for lenticulated color film and to enhance the projection of such pictures as to the efiiciency of illumination and the correctness of color reproduction.

LThis invention therefore may be described as having the object of raising the efliciency of illumination and reducing the consumption of electrical energy in the light source, inasmuch as the invention avoids the waste of light encountered in the color filters.

Another object of this invention is to enhance 'the quality of color reproduction by elimination of the harmful influence of temporary or local variation or of a displacement of the electric arc, thus producing colored pictures without color dominants.

Furthermore the invention will increase the brightness of projected pictures. as the invention removes the additional thermal load on the film, caused by infra-red radiation emanating from the light source, thus permitting full exploitation of the thermal capacity of the filrrl by the visible light only.

Still another object of this invention is the full utilization of the light flux emanating from the light source, as the invention provides means for an appropriate adaption of the illumination beam to the aperture of the projection objective.

Embodiments of this invention will be described in the following, reference being made to the accompanying drawings, where Fig. 1 illustrates schematically an illumination system with filter set accordingly to the invention, with a parallel arrangement of color filters,

Fig. 2 illustrates schematically a similar illumination system with filter set of reduced size,

Figs. 3, 3a, 3b and 3c illustrate diagrammatically the form, sectional area and aperture angies of light beams produced by such systems,

Figs. 4, 4a and 4b schematically illustrate an additional optical system as set forth by the invention employed to transform the sectional area of light beam before entering the filter set,

Fig. Lic schematically illustrates an equivalent optical-system employing glass solids of total refieotion type in place of parallel mirrors,

Fig. 5 schematically illustrates another filter set according to the invention with a parallel arrangement of color filters,

Fig. 6 schematically illustrates a different embodiment of the invention with crosswise arranged filters,

Fig. 7 schematically shows an illumination system producing a telecentric light beam,

Figs. 8 and 8a schematically illustrate lter sets with different types of additional optical means for equalizing the optical path length for the decomposedparts of the lig ht,

Figs.. 9' .and '921i and Fig. 9b, schematicallt7 illustrate, respectively, other` embodiments of optical equalizers for a filter set with cross-wise arranged lters,

Fig. 10 diagrammatically indicates.' the required optical characteristics of the reflexion `filters off a parallel set,

Fig. 11 diagrammatically a. parallel set with bent axis,

indicates the requiredoptical characteristics of the reflexion ltersrofv `with Ia lamp-sided position *ofthe color lter,

i. e. if the lter is inserted into the light path between the light source and the picture window. Only then all parts of the decomposed light flux can fully beiutilized. If the lter is inserted into such parts'where' the light has already passed the iilm and. is consequently impregnated with the 'recording of the' color components of the picture, the spectral partsof the light rejected by the reflexion lterin the light beams pertaining to In Fig. 1, light emanating from light source l', preferably the positive crater of an electric arc,

is projected by a parabolic mirror 2 into the entrance of a color lterset composed of reflexion filters 3 and 4 and a. mirror 5. .The light, issuing from the .'lter-set ,isrprojected onto, the .picture window 'I by a'l'ens E, which produces 'an image ofthe light source ...I Zonsaid picture Window. Thev reilexion flltersof'vthelset Aare preferably arranged to form an angle of 45? with theindent light, as-this .permits van optically correct separationforthe trans n i'itted and of the reeotedlpartsofV the light. The reexion -i-llter ,3.may. e. g. transmit the A'red components-of the incident white light whereas the green and .blue

components are reilectedlupwards atan angle ,o f ,.90,"l..to the originaluirectionof .the beam.

Ihisgreflected light. falls upon` a second reflexion .nit/er.. 4 .transmitting bluefandreflecting. green.

The bluelight issuingllfromthe lter 4` isV deiiectedby mirror 5-to,be .again parallel with the original axis of the light beam and with the. red andgreen beam issuing from -the iilterset. By

Ylens 6 V.images of the light source are formed vfromv'the. 'threev beams of different color. TheA respective images, however, are superposed in the plane ofl'the-picturelwindow"l, Y

-Byfth -employment of 'a parabolicmirror-.2, the light beam traversing the iilteihsetfis made telecentr-ic. A .light-beamis/spoken of as--tele- .centric iithe imageof ythe light source, as here .seen from the entrance of. the iilter,V appears .as lying atl any innitedistance. Irvadditionally thelemitting surfaceiof the light source-is small in comparison'tothe foca-l lengthfof the mirror y .all .light raysv of. the entire` telecentric v,beam` issuing from the reilectingsurface ofthe mirror Willbe approximately parallel to each-other. vThe insertion of a reexion lter into a telecentric y light beam of practically* parallel lightisespecially advantageous, as the losses produced inside `the lter set yby-eventualv mutualshading Aof the Viilters.are. ..very small. Furthermore the characteristicslof transmission and r'elexionf of a-*reflexion-lter depend` upon the direction in which very'narrow and the efiiciency of color separationis very high.

lIt is' evident that reilexion lters permit to obtaia high efficiency' of.' illumination as the light is split' into f'several' spectral 'components 'that all canfbe utilizedseparatelyx; On the conltrary-, absorptionfilters' suppress the not transmitted parts of the light; which causes ,a thermal charge.- ofA :the filter. fWith reexion 'filters an eiciency of illumination may be.attained. up to 90%, compared with a maximum .efficiency 9fthe different colors are worthless, as they are impregnated with the recording, and must be eliminated from the projection light path. Consequently in this objective-sided position, re-

flexion filters oier no advantages over; absorptionlters.' The advantages of a lanlp-sidedl positionfOf the color lter in .a projection; system vforcolored motionpictures can however lbe] fully lexploited, Cilly' '.by. theV employmentV of doublyl lenticulated Such doubly lenticulatedffilm has been describedinmy U. S. patent applica;- tion SergfNoJlil of'February- 23; 1950', and a projection systememploying suchdoubly',` lenticulated nlm, has been described in my U. S. patentl application. Ser. No.lgi5,'662 of February 23, 1950;',I has been explained inthe 'aforementioned applications only the employment of such 'doubly lenticulate'd film will permit to wholly utilize the fullgamount of .light components produced by an illumination systemas set forthby.thisinvention. f The employmentpoff'reflexiondfilters presents f still another advantage. .The partialv light beams Lisnot met bythe employment of vabsorption filters, as 'such filtersf abstract thepartial beams from different parts ofthe original beam of -white light. Such a divergingand heterogeneous distribution of light intensity.inthepartial. beams .is caused byadisplacement ofthe light source in respect to the focal point of the .light condensing means. Ijhe `sameis valid- :if 4.the in vtens'ityofflight varies across the light emanating `face `of saidisource.y Inbothcasesfcolor dominants on the picture screen; ensue.;

The employment of reflexion filters ensures an identical .distribution `of light intensity. in .the partial beams of.. different color, as said beams are abstracted from theV white beam in an identical-way. Forcibly the distribution of light infte'nsity` in the partial beams is. identical..v T his may easily be checkedy by projecting a pieceof blank lm. If thedistribution of light isidentical. for the red, green and blue component,

Ythe screen must show anvuniformly white=co1or, Vwhereas relative alterations betweenv the distribution in 'the colored beams produce colordominants either all over` orin partia1-areas of-the screen.` A displacement of the light source away from its correct position or other variations causing 1 inhomogeneity of illuminationr in theV white yagrammaticaily shown in Fig. 4. 3% a telecentric bea-m 32 of approximately square beam. will with reflexion filters produce only a variation of the intensity of white light projected on the screen. If absorption filters are employed, the same variations will produce color dominants.

An arrangement, as shown in Fig. 1 will produce very unhandy dimensions of the lter set and of the condenser lens 6. The dimensions may be reduced by the employment of a light source producing a converging light beam as is represented in Fig. 2. 8 is the light source, 9 a concave mirror. A dispersing lens I 0 will produce a telecentric light beam within the lter set I I. I2 is the condenser lens, projecting the image of the light source 3 into the picture Windows I3. Though the diameter of the concave mirror 9 is equal to that of -mirror 2 shown in Fig. 1, the dimensions of the lter set II and of the condenser lens I2 have been substantially reduced.

In order that the aperture of the objective projecting the nlm images on the screen be fully utilized, the aperture of the illuminating light beam must correspond to the aperture of the projection objective. In Fig. 3 I4 is the light source with the concave mirror I5, I6 the reflexion filter set, II the condenser lens, projecting an image of the light source into the picture Window I8, thus illuminating a piece of film situated in said picture Window. The film picture issuing from the filter and composed of the three color components will form a cone with rectangular base with a side ratio of 1:3. Such a beam only partly utilizes the aperture of the objective I9 which iorms a circular cone. y Fig. 3a shows the face of the objective 215 and the cross-section 25 of the light beam. A better utilization of the aperture of the objective would ensue, if the cross-section of the light cone could be made square. This may be realised if the entrance of the filter set I5 is deformed in the inverse sense, as shown in diagram 3b. The ensuing illumination beam 28 shown in Fig. 3c now efficiently utilizes the aperture 26 of the projection objective. if, however, the entrance of the lter set deformed in the above mentioned way is illuminated by the light beam of circular shape 22 shown in Fig. 3b, a considerable loss of light is produced.

' lOpticalmcans producing a light beam appropriate to the deformed entrance 2T of the color iilter without substantial loss of light are di- By a diaphragm form is cut out from a circular beam 34| produced by the light source as described above. This light beam 32 with square sectional area is transformed into a beam 33 of rectangular sectional Aarea .by the insertion o several sets of parallel mirrors into the light path. The beam 32 is divided into four identical beams 34, 35, 35, 3T. The partial beams 3S, 27 are displaced in horizontal vdirection by an amount equal to the side length of their sectional area by two sets of.

. parallel mirrors 38 and 3S displacing beam 3B and 3l respectively. This displacement is indicated schematically by arrows @D and 4l in Fig. .4a.

From this circle a square light s The beams 34, 35 are displaced by the same amount in vertical direction by a set of parallel mirrors 42. As indicated by arrows l113 and 44 they lit into the space left free by the lateral displacement of beams 36 and 3l. Thus out of the originally square light beam 32 a still telecentric light beam 33 of rectangular sectional area 33 with a side ratio 1:4 is formed. This beam 33 enters the lter set I6. The emerging three beams of colored light are projected into the picture window I8 by lens I'I, filling a cone 24 of rectangular sectional area of a side ratio of 3:4. `In order to reduce losses in the reflecting surface of the parallel mirrors 38, 39 and 42 glass solids employing total reflection can be used. The entrance surfaces of such prisms may then be coated by the generally known processes to avoid losses caused by reflexion. The aperture angle or the projection objective I9 may be utilized in a still more enicient way, if the cone 24 issuing by lens l is given a square sectional area. This is obtained by giving a side ratio of 4:3 to the beam 32 cut out by diaphragm 30. The losses of light, encountered vby cutting oliE parts of the circular beam by diaphragm are practically equal if the sectional area of beam 32 possesses a side ratio of 1:1 (Fig. 4a) or of 4:3 (Fig. 4b).

1n Fig. l one embodiment of this invention has been described.. This embodiment will be spoken of as set of parallel iilters. The obtainable number of color components naturally is not limited to three. By an appropriate choice of the transmission and reflexion range of the employed reflexion iilters an optional number of beams of diierent color may be separated.

Fig. 5 shows another form of such a set of parallel lters. Here a telecentric light beam l5, coming from a light source as described above, enters the filter from underneath. The issuing colored beams 46, 4l, 48 are at right angles to the entering beam 45. 'Such a deliection of the light path between the light source and the picture window may be of importance in certain cases.

lf the white light is to be split into only three color components another` embodiment may be employed as shown in Fig. 6. This embodiment will be spoken of hereinafter as set of crossed lters, as the filter surfaces cross at right angles. The telecentric beam 49 or" white light, produced in an analogous arrangement of light source and parabolic mirror as above, falls onto two filters 5t and 5I intersecting at right angles and inclined against the axis of the light beam 49 by an angle of 45. The filter surface 50 transmits green and blue the red part of the light is deflected upwards by an angle of whereas by filter 5i the green part is deliected downwards by 90 and the red and blue components are transmitted.

To study the effect of a set of crossed filters as described in Fig. 6 a light ray 58 will be followed. Said ray of white light coming from the light source comprises all color components. The red portion ci' the spectrum is reflected downwards by -.filter surface 5I by an angle of 90, traverses the filter if? and is again deflectedby 90 by mirror 5a. It leaves the iilter set parallel tothe original direction. of the light ray 58. The green and blue portions of bea-m 49 are transmitted by nlter 5 I. By the filter 50 the green portion is reiected upwards and deiiected by mirror 53 to be again parallel to the original ray, Whereas the blue portion traverses ilters 50 and 5I, issuing from the filter set parallel to the direction of the original ray.

L,7 I In order to avoid color dominants, it is necessary', as mentioned above, that the separated lightl beams though of different color .show an identical distribution of light` intensity.

This can only be realised, if the optical means producing such three components from the originally -white light beam provide equal optical path lengths, as the distribution of light intensity over a cross-section of a telecentric light beam varies with the distance of this section from the mirror. This will be explained by Fig. '1.

A light source 62 is situated in the focal point of a. parabolic mirror 60. Thereby atelecentric light beam 63 is produced. yThe light beam 63 issuing vvfrom the refiecting surface of mirror 60 is made up from elementary light cones, e; g. 59.', 59 and 59". The distribution of light in any cross-section of the light beam 63 corresponds to the ever increasing overlapping of such light cones as the distance of said section from the mirror 60 increases. Consequently vthe curves of distribution of light intensity inside a telecentric beam vary with said distance and assume a similar form only at a certain distance from the mirror, i.e. if the diameter off the emanating light beam S3 has grown to be several times greater than the diameter of the parabolic mirror (il).Y

As the reflexion lter is' inserted into the path of the telecentric beam with a distance from the .light source amounting only to a fraction of said distance the distribution of light intensity ,in any section of the beamvaries according to the optical path length between said section and the mirror. The sections of separated partial beams of different color vtherefore show identical distribution of light, if the length of the optical path length'between said section and the mirror is equal.

If the optical arrangement of the filter set does not correspond to this requirement, an equalisation of the length of respective optical paths can be effected by the employment of `additional optical equalisers. Thisis shown in Fig. 8 for a set of crossed filters according to Fig. 5. The two light beams, indicated by. rays 64` and 65 must be elongated by once or twice theidistance 61. This is effected by the insertion of sets of angular rmirrors into the light path of rays 64 and v65. The additional path in such angular' mirrors is made equal to the necessary velongation of the optical paths.

` The additional distance of the angular mirrors 68 is equal to the double distance 61, whereas the additional pathin thevangular mirror 69 is equal to that distance; Consequently both rays 64 and 65 are elongated by lan appropriate additional optical path `torequalthe optical path length of ray 66; The distribution of light intensity of the three light beams in plane 10 is forcibly identical across the whole section/of any of such beams, lasv the-distance between said plane and the light source is equal for all three beams.

The same effect can be obtained in a set of crossed reflexion filters according to Fig. 9.

`Here, howeven'only the path length of the middle ray 12 must be elongated, as the paths oftheside rays 1l and 13 already are of equal length. A set 14 of angular mirrors is' ragain inserted into thelight path of thev middle ray 12vv as optical equaliser. The plane in which the ray 12 is deflected and redeflected is, however, atright angles to the plane of defiection of the reflexion filter. This is required, if the set of angular mirrors 114 is not to interfere with the light paths of the other rays 1| and 13. Fig.: 9

is determined by:

l15 is a high-pass.

8 shows a. side view andAFig. 9a a bottomwlew of the selfsame embodiment. Y f Y In principle such optical elongation of light beams may valso be effected by the insertion -of a medium of higher refraction index into the light path.l Liquids possessing a very .high refraction Vindex are of particular importance. If n be the refraction index of-a medium and the proposed elongation be a, the necessary path length of the medium along the l-ightvpathfL of sequence of colors and of the position inthe frequency range of transmitting andreflecting areas of the individual filters of one set. Re-

rexion filters alternating only oncev between transmission and reflexion in the whole light frequency spectrum-may be more easily produced and have higher efficiency than filters alternating several times between transmission and reflexion. Furthermore, lters transmitting radiation vof long wavelength and reflecting radiation of short wavelength can be more easily produced rthan. filters with the inverse qualities. Employing terms well known in the art of electrical engineering, a iilter transmitting light of long wavelength and reflecting light of short wavelength will be spoken of in' the followingV as lowpass filter. light. of short wavelength and reflecting light of long wavelength as high-pass filter. A bandpass Vfilter will transmit medium wavelength and reflect long andshort waves of light, whereas a band-rejection filter` has the inverse effect. Y

O-n the other hand light sources with a high intensity of light require elimination of the infrared radiation contained in the white light. This requirement can be met by an appropriate choice of the sequence of colors of the filters ,of the set. Taking into account both points the sequence of color indicated as most advantageous for the described embodiments is indicated `in the following. Y

In order to discern more easily the components of color, separated by the light filter in the drawings referred to hereinafter a full line indicates red light, a dotted line green light, and an intermittent line blue light.Y

In the set of reflexion lters with straight light path shown in Fig. 10 the infrared radiation is eliminated without additional filters if' highpass'es and low-passes are employed. Filter'15 reflects infra-red, red and green, whereas blue is transmitted. Filter 16 reflects green (and blue) and transmits red and infra-red, andV filterv11 reflects red (green and blue) and transmits infrared. vFilters 1,6 and 11 are low-passes vand All three filters only have one alternating point between transmission and reflexion all over the whole infrared and visible spectrum. Y

In a parallel lter set with deflected light path only low-passes areV required (Fig. 11) and still the infrared radiation may be eliminated. Filter 18 reflects blue and transmits infrared, red and green. Filter 19 reflects green (and'blue) and transmits red and infrared,v whereas filter In contrary a filter 'transmitting reflects red green and blue) andtransmits infrared. As only low-passes are required, transmitting long-waved vand reflecting short-waved light, this embodiment of a filter offers the least difficulties for production. `Two optical equalizers, however, are necessary to obtain light beams of equal optic path length behind the lter set.

In a set of crossed filters the outer beams already comply to the requirement of equal optical length of the light path. An optical equalizer is required only for the middle beam. Often even this optical equalizer may be disposed of. The requirement of identical distribution of light intensity across the whole light beam is more rigid as regards the red and green beams than the blue one. Slight Variations of light distribution within the blue beam practically do not impair the quality of reproduction, if only the red and green beams are identical all over the screen. If the green andv red components are producedrby the outer light paths of the set of crossedelters, both will be optically identical and no optical equalizer will be required for the middle path. v

An embodiment of a cross reflexion filter set is shown in Fig. 6. Filter reflects red and infrared and transmits green and blue. Filter 54 reflects red (and green and blue) and tran..- mits infrared. Filter 50 reflects green and transmits infrared, red and blue. Filter 53 reflects green, red and blue and is just an ordinary mirror. Filter 5| consequently is a high-pass. filter 54 a low-pass and illterl 50 a band-rejection-lter. This filter set consequently demands higher expenditures than a set of parallel filters. This, however, may be of minor importance than the advantage oifered by symmetrical outer beams.

If an optical equalizer 'M is inserted into the middle beam as shown in Fig. 9 providing three identical beams, the band-rejection lfilter can be avoided. Filter 85 reflects infrared and red, and transmits green and blue. Filter 81 reflects red (green and blue) and transmits infrared. .Filter BBtransmits green, red and infrared and reflects blue. Consequently filter 86 represents a highpass Whereas filter B1 and 88 represents lowpasses. This latter arrangement of a crossed filter-set avoids a band-rejectionlter.

As has been mentioned above with respect to the optical means shown by Fig. 4, glass solids employing total reflection may also be used in place of the parallel mirrors. This is shown by Fig. 4c, wherein identical parts are given the same reference numerals. The two sets of parallel mirrors 38 and 39 are `replaced by glass solids and 202 and the set of mirrors 42 is replaced by glass solid 203. A telecentric beam of parallel light is produced by lens 204 and passes through the square opening of diaphragm 30. The upper half Yof the beam impinges uponthe front surface of solid 203 and is reilected twice inside the solid by total reilection occurring at its surfaces 205-and,206. The'lower half of the beam impinges upon the front surfaces of solids 20| and 202 and is displaced towards both sides by total reflection occurring at surfaces. 201, 208, 209 and 2 |0 of said solids. The displaced beams issuing from the rear surfaces of the glass solids now form a rectangular beam haivng a section 33. The glass solids 20|, 202 and 203 employing total reflection thus produce .the same effect as the sets 38, 39 and 42 of parallel mirrors. .If the rectangular beam 33 is nowvmade to impinge upon the entrance of a filter set of parallel mir- CII rors, as shown by Fig. l0, and composed of twoA dichroic mirrors l5 and I6 and deflection mirror l1, the composite beam issuingfrom this filter set will have a cross section 2| which is apj aroximately` square and is capable of being fully utilized by the projection lens. If the transmission characteristics of mirrors 15 and H5 arey chosen as has been explained with respect to Fig. 10 a total beam will be composed by three beams of red, green and. blue color, the boundaries of which are contiguous at the exit of the filter set.

Glass solids employing total reflection may also be used to replace the sets of angular mirrors used to` obtain equal optical path length for the beams of different color. This is shown by'Fig. 9b, which shows la set of crossedmirrors equal to that shown by Fig. 9, and wherein equal elements are given the same reference' numerals. Ashas been explained above with a setof crossed mirrors composed of two intersecting dichroc mirrors and 81 and two lateral mirrors 8,8 and 89, only the center beam must be elongated. In place of vthe angular mirrors 'I4 shown in Fig. 9, a glass solid 220 is inserted into the light path cf the green'center beam. The beam enter-` ing through the front surfacer 22| is successively reflected by surfaces 222, 223, 224 andj 225;` By asuitable design of the glass solid 220 the additional path length .of the lightv beam inside the glass solid may be made equal to the' additional path length'of the lateral beams, 'thereby the optical path length between the section 22B cf the original beam of white light and of a section 221 through the .composite beam will be equal for all partial beams. y o

As has beenm'entioned above, optical elongation of the light beams may also be effected by insertion of a medium of higher refraction index into the light' path. This is shown by Fig. 8a which otherwise corresponds to Fig.l 8. 'In the set of parallel mirrors comprising dichroic mirrors 3 and A andv deflection mirror 5, the center beamy 65 must be .elongated by once, andthe lower beam 64 by twice the optically effective distance 6'! between the mirrors. For'this purpose a solid 230 is located in the light path of beamv 55 anda solid -23| in the light pathj'of beam 54. vThepath length. within thesesolids Amust now be chosen in themanner indicated above. The size of the solids shown by way of example in Fig. 8a corresponds tov a refraction index of 1.5. As has been mentioned above, also liquids may be employed which provide still higher indices Yof refraction, vand must then ,be inclosed in a glass receptacle. Thereby. likewise the optical path length between a section 2,32 through the light beam and vsection 233 through the composite beam may be made equal for a three partial beams l|54, 05 and 66.

While I have shown and described particular embodiments of my nvention` I however'do not wish to be limited thereto but desire the appended claims to be construed -as broadly as Apermissible in the View of prior art. Especially it is to be understood that whilst in the description mention was made only of telecentric beams, itis valid also for non-telecentric beams.

I claim: f

l. For yuse with apparatus for the projection of lenticulated film said apparatus being of the type including a light source anda picture window behind which a lenticulated nlm maybe positioned to be illuminatedby 'saidsource of light through said picture window, the combination ing 11 of'a color filter set of non-absorptivev type comprising a plurality'of dichroic mirrors consecutively positioned inthe light path from said light source to said Apicture window, said mirrors having different spectral band reflection-transmission characteristics and developing from incident light from said source geometrically separate beams of different colors of a total lightenelgy content'substantially equal to the lightenergy contentof said'incident light, and auxiliary optical means for directing saidseparate beams in coincidence upon the said picture window.

2,. A fllter'set for the projection of Vlenticulated films, and forV use with projection apparatus including a sourcev of white light and a projection windowbehind which a lenticulated film may be positioned; said filter setbeing of non-absorptive typeand comprising a plurality of dichroic mirrcrs spaced apartand consecutively positioned in the lightpath from said white light source to said picture window, each of said mirrors extending fully across the saidlight path and having'a'n active optical surface which reflects one spectral band of the incident light and transmits the remaining'. spectral bands of the incident light,thereby to develop a plurality of geometric'allyiseparate beams of differentcolorshaving atotallight-energy content substantially equal to the light-energy` of said white light source, and optical means for directing the separate beamsin coincidence upon said picture window. 3,. The combination set forth in claim 2, where the. first dichroic mirror nearer tov the light source transmits blue light and reflects green and red light, and where a seconddichroic mirror farther away from the light source transmits red and reflects green and blue light.

4. The combination as vrecited in claim 3, in combination withja mirror for reflecting towards sraidfwindow. opening the light beam transmitted by; said second reflection filter.

The combination' .as recited in claim 4, wherein said mirror vis a dichroic mirror reflectvisible; rays and transmitting infra-redrays.

6, Inl apparatus for the projection of lenticulated film', alight source, a picture window behindwhichajlenticulated film may be positioned to'b'e' illuminated by said source of light through A saidf'pieturel window.' al color lter set inserted intoY thel light path between said source of light and' said .picture window,V producing light beams 0jf different color required4 for the projectionl of pictures recorded onV saidlenticulated film, and Y additional optical means inserted intothe light path'. between said source. and said picture window causing the light emanating from said source to traverse said ltjer set and subsequently to converge against said picture window; said filter; set comprising two dichroic mirrors with their optically active surfaces approximately parallel to'each other and at an angle of 45 to the incident light, and an additional mirror located Vbehind said second dichroicv mirror in the. direction yof' the transmitted light and approximately parallel to.` said active surfaces of the dichroic mirror. 7

'7. The combination set forth in claim. 6;.where the; beam of white iight enters-the nnerv set at right angles vto the direction of the produced beams of coloredl light, the first dichroic mirror nearer to the light source transmits green and red light andy reflects blue light, and where the second dichroic mirror farther away from the light sourcetransmits red and reflects green and blue'light. I i I y 8'. The combinationY asA recited 'in claim V'1, wherein said additional mirror is aV dichroic mirror reflecting visible rays and transmitting infrared rays. v

9. In apparatus for the projection of lenticulated film, a light source, a picturefwindow behind which alenticulated film may be positioned to be illuminated by said source of light' through said picture window, av color filter Vset inserted into the light path between said source of light and said picture window producingv lightbeams of different color required for the projection of pictures recorded on said lenticulatedv film, optical'means inserted into the light path between said source and said picture window causing the light emanating from said' source to traverse said filter set, and additional optical meansto converge the4 beams of different color upon'said picture window; said filter set'comprising two dichroic mirrors arranged with their-'optically active surfaces intersecting at an angle of and inclinedv by yan angle of 45 to the direction of the incident light, and two additional mirrors each of which is arranged in the path of light reflected by one of said mirrors and parallel to the active surface of the dichroic mirror from which light isv reflected to it whereby the beams ofy colored light reflected by such mirrors are again made parallel with the beam of white light entering the filter set and with the beams of colored lightv transmitted by the dichroic mirrors.

l0. The combination set forth, in claim 9, where ,one of said dichroic mirrors transmits blue and red light' and reflects green light, and where the other dichroic mirror transmits blue and green light and reflects red light. o

11. The combination set forth in claim 9, where one of said dichroic mirrors transmits red and green light and reflects blue light,A and where the other dichroic. mirror transmits blue and green light and reflects red light.

12. The invention as set forth in claim 1, in combination with optical means to produce from said light source a telecentric light beam inside the filter set.

13. The combination set forth in claim l2, and where said optical means comprise a dispersing lens inserted, between the light sourceV andthe filter set producing a telecentric light beam inside said filter set and; saidv auxiliary optical meansrcomprises a ycondenser lens inserted between the filter set and the` picture window causing the light beams issuing from the filter set to converge againstthe picture-window.`

14. The combinationset forth in claim ll where the aperture of theentranceof said filter 4 set" is of rectangular shape with a ratio of the sides parallel to the filterl zones to the sides at right, angles to the'fllterzones exceeding 1.5:1; and' wherein an additional optical means develops from a homogeneousbeam of light from'said source and` ofy substantially circularcross-section a homogeneous beam of light ofthe rectangular cross-section of said entrance aperture.

lgoThe'vcombination set forth incl'aim 14, where-.said additinaloptical means comprise one Y additional angular mirrorl systemv inserted intoon'epart ofxthe light Abeam between the light L source 'andgthe'filter set, splitting the'light pertainingrto. said part of4 'light beam into two partial beams andk displacing each of said partial beams sideways. in'V Vopposite directions byv approximately their'breadth, and vanother addi- 13 tional angular mirror system inserted into the remaining part of the light beam between the light source and the filter set, displaicng the partial light beam pertaining to said remaining part of the light beam as to Iit into the space left free by the displacement of the two iirst named partial light beams.

16. The combination set :forth in claim 1, in combination with means including an angular mirror system inserted into the light paths of the colored light beams issuing from the iilter set to produce inside the filter equal optical path lengths of all beams.

17. The combination set forth in claim 16, Where glass solids employing total reiiection are used to constitute said angular mirrors.

18. 'Ihe combination set forth in claim 1, in combination with means including a material of high refraction index in the light path of a colored beam for equalizing the length of the light paths of all of said separate beams.

19. The combination set forth in claim l, wherein one of said reflection filters selectively transmits and reflects infra-red and visible rays respectively, whereby the infra-red radiation is eliminated from the path of the projection light beams.

20. The combination set forth in claim 2, wherein one of said reection filters reilects red, but transmits infra-red light.

21. The combination set forth in claim 6, where said additional mirror reflects red, but transmits infra-red light.

22. The combination set forth in claim 9, where one of said mirrors reflects visible light but transmits infra-red light.

23. In a projection apparatus for lenticular color 111m and of the type including an illumination source developing a beam of white light of substantially square cross-section, a picture Window of rectangular form at which a iilm may be illuminated by said source, and a filter set between said source and said picture Window for directing geometrically separate beams of different color light upon said picture window; an optical system between said illumination source and said lter set including means for splitting a portion of the white light beam to form two partial beams, means for displacing the partial beams in opposite directions from their original positions, and means for displacing another portion of the white light beam into the space left free by the displacement of said partial beams.

24. In a projection apparatus, the invention as recited in claim 23, wherein said means for forming two partial beams develops partial beams of rectangular cross-section.

25. In a projection apparatus. the invention as recited in claim 23, wherein said optical system comprises glass solids of total reflection type.

EDGAR GRE'I'ENER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 944,787 Jameaux Dec. 28, 1909 973,962 Oliver Oct. 25, 1910 1,163,192 Adams Dec. 7, 1915 1,216,835 Morris Feb. 20, 1917 1,430,494 Correll Sept. 26, 1922 1,599,719 Roach Sept. 14, 1926 1,607,661 Albert Nov. 23, 1926 1,817,026 Wright Aug. 4, 1931 1,986,425 Ceccarini Jan. 1, 1935 1,988,882 Thomas Jan. 22, 1935 2,027,369 Bourges Jan. 14, 1936 2,090,441 Eggert et al Aug.l 17, 1937 2,201,487 Gretener May 2l, 1940 FOREIGN PATENTS Number Country Date 7,179 Great Britain of 1904 206,820 Great Britain June 11, 1923 94,088 Austria Aug. 25, 1923 

